<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let r=prompt("请输入半径");
        let Pi=3.14;
        let S=Pi*r*r;
        Document.write("圆面积是"+S);
    </script>
</body>
</html> 